Separation membranes are utilized in various fields, for example, the field of water treatment such as manufacturing of drinking water, water purification processing, and effluent treatment; the field of fermentation in which production of substances accompanied with cultivation of microorganisms or cultured cells is carried out; and the field of the food industry. In the field of water treatment such as manufacturing of drinking water, water purification processing, and effluent treatment, the separation membranes are used so as to eliminate impurities in the water, as substitutions of the sand filtration and the coagulation settling process in the background art.
There are various types of separation membrane modules, and the generally known technology uses hollow fiber membrane modules in which an installation area per membrane area is small and replacement cost for the separation membrane modules is inexpensive. As a filtration method, there are (1) a total amount filtration method in which filtration is performed by supplying a filtration target liquid to the separation membrane modules, and (2) a cross-flow filtration method in which a filtration target liquid is supplied to the separation membrane modules, a portion thereof is filtrated, and most of other portions thereof are caused to circulate to a storage tank or the like of the filtration target liquid. In the cross-flow filtration, an effect of removing sediments on a separation membrane surface by the shearing force of a stream of cross-flow which is parallel to separation membranes can be expected. Therefore, the cross-flow filtration is preferably used when processing a target liquid having high turbidity concentration.
As an application example of filtration technology using the separation membranes, a continuous fermentation method can be exemplified. In other words, according to the continuous fermentation method, it is proposed that microorganisms or cultured cells are filtrated through the separation membranes, thereby collecting a chemical from a filtrated liquid, and at the same time, retaining the microorganisms or the cultured cells in a fermentation cultured liquid or refluxing the microorganisms or the cultured cells in a concentrated liquid to the fermentation cultured liquid. In accordance with the method, the concentration of microorganisms or cultured cells in the fermentation cultured liquid can be maintained to be high.
With regard to a continuous-fermentation apparatus, a technology has been disclosed in order to perform the production through more efficient continuous-fermentation. The technology uses hollow fiber membrane modules in which an installation area per membrane area is small and replacement cost for the separation membrane modules is inexpensive (refer to Patent Document 1). According to the technology using the hollow fiber membranes as separation membranes, the concentration of microorganisms or cultured cells in the fermentation cultured liquid can be maintained to be high by collecting a chemical from a filtrated liquid, and at the same time, retaining the microorganisms or the cultured cells in a fermentation cultured liquid or refluxing the microorganisms or the cultured cells in a concentrated liquid to the fermentation cultured liquid. The technology employs cross-flow filtration in which a fermentation cultured liquid is supplied to the hollow fiber membrane modules, a portion thereof is filtrated, and most of other portions thereof are refluxed to a fermentor. Contaminants on a membrane surface can be removed by the shearing force of a stream of cross-flow, and thus, efficient filtration can be continued for a long period of time.
Here, in continuous fermentation which is carried out on the industrial scale, a large fermentor is used, and volume thereof is assumed to be several hundred m3. A significant membrane area is necessary in order to perform filtration with a fermentation liquid in large quantity including highly concentrated microorganisms. In order to realize the significant membrane area, it is effective to use a plurality of the separation membrane modules with respect to one fermentor. For example, when a fermentation liquid of hundred m3 is filtrated, hundreds or thousands of the plurality of separation membrane modules are used at most. However, the most suitable number of the separation membrane modules may vary depending on filtration properties of the fermentation liquid and performance of the separation membrane modules.
According to Patent Document 2, cross-flow filtration is employed in order to obtain an effect of cleaning the separation membranes. From a view point of reduction of operational cost, the separation membrane modules are disposed in series so that the flow rate of cross-flow can be reduced (Patent Document 2).
Since a large amount of water is processed also in the field of water treatment such as manufacturing of drinking water, water purification processing, and effluent treatment, many separation membrane modules are used. There is a disclosed technology in which the separation membrane modules for cross-flow filtration are disposed in series so as to reduce the used amount of a raw liquid and the processing amount of waste liquid when flushing cleaning is performed by causing the raw liquid to flow outside the hollow fiber membranes (Patent Document 3).